Generally, semiconductor devices are fabricated by means of complex processes such as deposition processes where thin films are deposited on a substrate, photolithographic processes where photoresist patterns are formed on the substrate, ion implantation processes where impurities are implanted into the substrate, and etching processes where specific layers are removed. The etching process may include a dry etching process and a wet etching process. In the dry etching process, plasma is produced in a vacuum chamber to remove a thin film formed on a substrate surface. In the wet etching process, a substrate is submerged into a chemical bath to remove a thin film. A spin etcher was recently introduced, in which a vaporized etchant is sprayed onto an upper side, a lower side or an edge of a rotating substrate to etch a specific area of the substrate. Since the spin etcher has an excellent area selectivity and easily controls an etch thickness of a thin film, it is widely used to etch a thin film. Here the characteristic of “area selectivity” means an ability to etch only a specific area. Therefore, in order to adjust a thickness of a thin film, it is very significant to measure an etching end point.
Conventionally, gas elements produced as etch by-products are analyzed using a spectrometer to detect an etching end point. In this case, an etching amount error may occur due to variations in etchant concentration and variations in process conditions. The etching amount error makes it difficult to exactly adjust an etching thickness.